Pickup device



Feb. 4, 1969 o. H. SCHADE, SR 3,426,235

PICKUP DEVICE Filed Dec. 20, 1966 0770 /16 JZ/Mg J2e 3,426,235 PICKUP DEVICE Otto H. Schade, Sr., West Caldwell, NJ., assigner to Radio Corporation of America, a corporation of Delaware Filed Dec. Ztl, 1966, Ser. No. 603,266

U.S. Cl. 315-11 12 Ciaims Int. Cl. H0137 3.7/48

ABSTRACT F THE DSCLSURE A high resolution vidicon type pickup device includes a two-gun assembly comprising two off-axis cathodes having emitting surfaces tilted with respect to the longitudinal axis of the device so as to cause electron beams originating at such surfaces to be in tangent relation with respect to magnetic tlux lines produced by a focusing coil to prevent spiralling of the beam. One gun is a reading gun and the other gun is an erase gun.

My invention relates to pickup devices and particularly to an improved gun structure for enhancing the operation of sucth devices.

While my invention may be employed, for example, in any tube having a storage target associated with reading beam means and in other devices having two electron guns, it will be Adescribed in the following in connection with a vidicon.

A vidicon comprises an evacuated tubular envelope enclosing an electron gun and a photoconductive target electrode. The target electrode includes a conductive coating or signal electrode on the gun side of a transparent support such as a faceplate of the tube envelope. The target electrode also includes a photoconductor comprising a layer of photoconductive material on the transparent conductive coating. Photoconductive materials are characterized by a change in their electrical conductivity in response to incident radiations. When in the ldark, these materials have a relatively high electrical resistance; and when exposed to light or other radiations of a selected frequency, they yacquire a relatively high electrical conductivity. Closely spaced from the surface of the photoconductive material, on the side thereof that is exposed to the electron beam, is a beam decelerating electrode in the form of a iine mesh screen.

For high denition, i.e., sharpness of the image formed on the target, and high resolution, i.e., fine detail, it is desirable that the target storage surface be characterized -by relatively high capacitance and that the reading beam have a relatively small cross section. However, such high capacitance storage surface and thinness of the reading beam are accompanied by the difficulty of completely discharging the storage surface with the reading vbeam after the surface has been scanned by the beam. The high capacitive lag characteristic of high capacitance storage targets causes carryover of the residual image into succeeding frames with a consequent smearing of the picture transmitted by the tube for moving subjects.

Accordingly, lan object of my invention is to provide an improved pickup tube.

A further object is to provide a pickup tube for use in monochrome or color cameras, that is characterized by relatively high definition and resolution and improved signal-to-noise natio.

Another object of my invention is to completely discharge a high capacity storage target of a pickup tube after completion of each horizontal scanning line with- States Patent 3,426,235 Patented Feb. 4, 1969 JCC out encroaching on the time required for carrying out the horizontal scans of the target.

One example of a camera tube in which the foregoing objects are realized, is a vidicon having a target including a relatively thin photoconductive layer for increased capacitance. The relatively thin character of the photoconductive layer is so related to the target area of the tube that with increase in such area the thickness of the photoconductive layer may be increased without reducing the desired high capacitance of the target.

The high capacitance of the target is of advantage in that it contributes to the formation of a relatively large electron charge on the surface of the target facing the electron gun. Such large charge concentration is desirable for high definition.

It is fortuitous that the thinness of the photoconductive layer resulting in a relatively high capacitance for realizing the aforementioned high electron charge concentration, is also of advantage in increasing the resistance of the photoconductor to a lateral spread of the electron charges thereon. Such resistance to lateral spread of the charges contributes substantially to increased [definition and resolution of the target.

While a high capacitance target contributes to high definition, it is characterized by an objectionably long capacitive lag. Such long lag gives rise to relatively large residual ciharges in the photoconductor in fast exposure readout cycles. In such fast readout it has not been feasible heretofore to completely erase the target with the reading beam before the next scanning frame, because the length of a capacitive lag period may exceeld the period of a scanning frame considerably. A capacitife lag period is a length of time required to fully discharge the target. The situation in slow scan operation is not much better. While slow scan operation may provide time for an erase cycle between scans, the erase means is in the form of a small cross-section reading beam which requires a long erase time.

I effectively fully discharge a high capacitance photoconductive target, both in slow scan and in fast scan operations, by providing, in addition to the usual reading gun, a second gun having an erase function only. The second gun produces a beam having appreciably larger cross-section than the beam produced by the reading gun. The relatively large area erasing beam accomplishes a complete discharge of the residual charges in the photoconductive target within a relatively short time interval with consequent enhancement of tube operation.

In order to accommodate the two guns in a limited space in a tube envelope, I provide a novel and advantageous gun assembly in which the cathodes only, of the two guns are separate structures. The other elements of the guns such as the grids and limiting aperture plates are common with respect to both guns.

Where a focusing coil is employed for focusing the beams, I position the cathodes so that the axes of their emitting surfaces converge with respect to the tube axis. The degree of convergence is such as to dispose the beams from the cathodes in tangent relation with two magnetic lines of flux produced by the focusing coil. Such tangent relation precludes a cutting of a line of magnetic -ux by the beams and therefore preserves the beams from a spiraling motion and consequent failure to land on the target. Such spiraling motion would be induced were a beam to intersect a line of magnetic flux.

In an operating camera the image surface is scanned from bottom to the top. I mount the reading beam above the erase beam, so that on completion of a scanning line by the reading beam Aand during the retrace period, the erase gun is energized for removing residual charges in the lines which have just been traversed by the reading gun beam. During energization -of the erase gun, the reading beam is blanked to prevent it from landing on the storage surface. In effecting a scan by the beams of the two guns with respect to a Igiven line, I utilize only the blanking or retrace time of the reading beam for operation of the erase beam. Since this time is relatively short, it is desirable that the erase beam be provided with a relatively large cross section. I have found that for best results the ratio of the cross-section area of the erase Ibeam to that of the reading beam should be substantially larger than the ratio of a line scanning period to a blanking period. For example, if the retrace or blanking time is about one-tenth the time required to scan one line, the cross-sectional area of the erase beam should be at least about ten times as large as the reading beam and have a similar current density. To obtain a uniform current density in the erase beam, I provide a wire grid or mesh in the path of the erase beam adjacent to the cathode of the erase gun.

The cathodes of the reading and erase beams are sequentially impressed with suitable voltages required for their operation during their sequential scans. During operation, the cathode of one of the guns is impressed with a suitable voltage for electron emission, which may be ground potential or slightly negative with respect to gro-und, while the cathode of the other gun is impressed with a voltage positive with respect to ground for suppressing electron emission therefrom. The energization of the two cathodes may be controlled for sequential electron emission, by scanning and blanking circuits employed in the deflection system of the tube. If desired, the blanking of the reading beam can be effected by target blanking as well known in the art.

Further objects and features of my disclosed subject matter will Ibecome apparent as the present description continues.

In the drawing, to which reference is now made for an exemplary embodiment of my invention:

FIG. l is a side view partly in section of a vidicon tube with associated beam control coils, in which my disclosure is embodied;

FIG. 2 is an enlarged view in section taken along the line 2 2 of FIG. 1 of a two-gun beam-forming structure used in the tube shown in FIG. l;

FIG. 3 is a View in the direction indicated by the arrows 3--3 in FIG. 2 and shows a common grid structure associated with the two guns; and

FIG. 4 is an enlarged fragmentary view of the target depicted in FIG. 1 and shows the relative positions of lines sequentially scanned by the reading and erase guns, and the relative thickness of the beams formed by the two guns.

The pickup' tube 9 shown in FIG. 1 comprises an elongated envelope which may be made of glass for example. At one end of the envelope 10 is a faceplate 12 which also may -be made of glass. The faceplate 12 is sealed across one end of the envelope 10 by suitable means such as an indium ring 14. On the inner surface of faceplate 12 is a transparent signal electrode 16 comprising a light-transparent conducting layer made of rhodium for example and having a thickness of about 100 Angstrom units. Rhodium is preferred as the cornposition of the signal electrode 16 because it adheres well to the glass faceplate and contributes to adherence thereto of a photoconductive layer. Over the signal electrode 16 is a relatively thin photoconductive layer 18. The thinness and area of the photoconductive layer 18 are such as to impart a relatively high capacitance thereto for enhanced definition. For example, in a vidicon having a 41/2 inch target, the photoconductive layer 18 may have a thickness up to about 2 microns and preferably about 11/2 microns. In one example, the photoconductive layer 18 comprises a combination of antimony trisulfide and 4 antim-ony oxysulfide (ASOS) described in U.S. Patent 2,875,359 issued to A. D. Cope.

The other end of the tube 9 terminates in contact prongs 20 for suitably energizing electrodes within the tube, and in an exhaust tubulation 22 shown pinched off, through which the envelope is evacuated. Adjacent to the aforementioned other end of the tube 9 is mounted my novel gun assembly 24 which will be described more fully in connection with FIG. 2. Between the photoconductive layer 18 and the gun assembly 24 is mounted an elongated focusing electrode 26 having a mesh screen 28 across the end thereof adjacent to the photoconductive layer 18. The other end of the focusing electrode is open. The focusing electrode 26 is supported within the er1- velope 10 in known manner, the support means ybeing shown schematically in the form of clips 30.

Externally of the envelope 10 and coaxial therewith is an integral coil assembly 31 including a group of conventional coils comprising a focusing coil 32, a scanning coil 34, a beam convergence coil 36 and an alignment coil 38, all having structures encircling the tube envelope 10. All of these coils are above the gun assembly 24 as shown in FIG. l.

't In addition to these coils above the gun assembly 24 I provide an ladditional -coil 40 also including a structure .encircling the envelope 10 but disposed below the plane 4in which the upper end of the gun assembly 24 termimates, as viewed in FIG. 1. This position of the coil 40 is significant, since it is not intended that the eld of coil 40 affect the beams issuing from the upper end of the gun assembly. The function of coil 40 is to influence the magnetic lines of flux produced yby the focusing coil 32, so as to displace two of such flux lines disposed on 'opposite sides of the axis of tu'be 9, into positions wherein the two beams formed by the gun assembly are in tangent relation thereto. As shown in FIG. 1, the coil 40 may constitute a solenoid having 1000 wire turns 41 in one example. In operation, the coil 40 is connected to a power supply (not shown) in series with the coil 32. The power fed to the coil 40 may be suitably controlled by rheostat (not shown).

The coil assembly 31 is provided with an inturned flange 42 at the upper end thereof as viewed in FIG. l, for service as a stop for desirably orienting the tube 9 therewithin.

For reading and erasing information from the high capacitance photoconductive layer 18 in either slow or fast scan operations, I provide a novel electron gun assembly 24 for producing a reading beam of relatively small cross-sectional area for enhanced resolution and an erase beam of relatively large cross-sectional area for substantially completely erasing all residual charges remaining in the photoconductive layer 18 after readout, for improved operation.

My novel gun assembly 24 comprises elements of which two tubular cathodes only, are independent structures. One of the two cathodes 44 is associated with portions of grid and limiting aperture plate structures for providing an electron beam of relatively small diameter, i.e., l mil in the example under consideration. The other of the two cathodes 46 is in register with other portions of the grids and limiting aperture plate for producing an electron beam of relatively large diameter, i.e., 30 mils in the instant example.

Cathode 44 has a closed end 48, the outer surface of which is coated with electron emitting material 49 such as a mixture of the oxides of barium, strontium and calcium. The end 48 is tilted with respect to the axis 50 of the gun assembly so as to dispose the beam produced by the cathode 44 in tangent relation with respect to a magnetic flux line 52 formed by the ux coil 32 and oriented by the coil 40. Cathode 44 is supported by an insulating disc 54 made of ceramic for example and which is fixed as by brazing to the inner wall of a gun assembly cylinder 55. The disc 54 has a first opening through which the cathode 44 is xed. A suitable lead 56 feeds desired potentials to the cathode 44, as will be explained in the following.

A second cathode 46 is provided having a structure similar to that of cathode 44. Cathode -46 is fixed through a second opening through the insulating disc 54. The cathode `46 has a closed end 58 coated on the outside with emitting material 60 which may 'be similar to the material 49 of cathode 44. The end surface 58 of the cathode 46 is tilted with respect to the gun axis y50 so as to dispose the bea-m produced by the cathode in tangent relation with respect to a magnetic flux line 62 produced by the focusing coil 32. The flux line `62 is on a side of the gun axis 50 opposite to that of the aforementioned liux line 52. A lead 64 is connected to cathode 46 for impressing suitable potentials thereon.

Bothe of cathodes 44 and 46 have suitable heaters for heating the emitting materials 49, 60 to electron emitting temperatures. Heater 66 serves cathode 44 and heater 68 is associated with cathode 46.

The grid or beam control structure common to both of cathodes 44, 46 includes a grid structure 70 closest to the cathodes. The grid structure 70 has a cylindrical portion 71 lixed as by brazing to the insulating disc 54. The grid structure 70 also includes a portion 72 having an opening 74 of about 40 mils diameter in register with the cathode 44 and titlted with respect to the gun axis 50 to substantially the same degree as cathode end 48. The grid structure 70 also includes a portion 76 having an opening 78 which may have a diameter of about 50 mils. Grid portion 76 is tilted from the gun axis S0 in a manner similar to cathode end 58. To preserve a uniform current density in the appreciably wider beam passing through the opening 78, the opening is spanned by a group of equally spaced parallel wires 80 shown best in FIG. 3. In the instant example, four such wires are employed. Each of the wires 80 comprises a molybdenum core plated with gold. The wire diameter is about 0.6 mil. The wires 80 may be fixed as by brazing to the upper surface of grid portion 76 as shown in FIG. 3. If desired, the wires 80 may instead be fixed as by brazing to the lower surface of grid portion 76.

A metal shield 82 xed to the grid structure 70, extends between the emitting portions 49, 60 of the two cathodes 44, 46. The purpose of the shield is to prevent electron emission from one cathode to the other when the cathodes are cycled with appreciably different potentials, as -will be explained in the following.

A second grid 84 comprises a sheet metal structure having a diameter for engaging the inner walls of gun cylinder 55 and to which it is suitably fixed as by brazing. Grid 84 includes a portion 86 spaced about 100 mils from and parallel to portion 72 of the iirst grid 70. Grid portion 86 includes an aperture 88 having a diameter of about 50 mils and in register with the opening 74 in the rst grid 70, along an axis normal to cathode end 48. Grid 84 also includes a portion 90 spaced about 100 mils from and parallel to the grid portion 76 of the rst grid 70. Grid portion 90 has an aperture 92 of 50 mils diameter. The aperture 92 is in register with aperture 78 of the first grid 70 along an axis normal to the cathode end 58. A group of four wires 94, similar to wires 80 of the first grid 70, span the opening 92 and are fixed to either the upper or under surface of grid portion 90 as by brazing. Preferably, the wires in one grid are at right angles to the wires of the other grid y(not shown) A limiting aperture plate 96 extending normal to the gun axis 50 spans the upper end of gun cylinder 55 and is suitably fixed as by brazing to afliange 98 of the gun cylinder 5S. The aperture plate 96 is spaced about 300 mils from the plane of the periphery of the second grid 84, and has two limiting apertures 100 and 102. Limiting aperture 100 has a diameter of about l mil and is coaxial with apertures 74 and 88 of the rst and second grids, along the aforementioned axis normal to cathode end 48.

The limiting aperture 102 has a diameter of about 30 mils and is in axial register with openings 78 and 92 in the lirst and second grids respectively, along the previously referred to axis normal to cathode end 58.

The degree of tilt of cathode end surfaces 48, 58 with respect to the gun assembly axis 50 is determined empirically, to cause their electron beams 104, 106 to assume a tangent relation with respect to the two magnetic flux lines 52, 62 respectively, produced by the focusing coil 32. In one example, this degree of tilt is such as to cause the electron vbeams 104, 106 to converge equally towards the gun axis 50. The degree of convergence of each of the beams with respect to the gun axis 50` in this example is such that the angles X and Y (FIG. 2) each have a magnitude of 5 40. In effecting this empirical determina ton, the location of linx lines S2, `62 is plotted with the use of a flux meter such as a Bell Gauss meter available commercially.

In the event the empirically determined cathode tilt with respect to the gun axis 50, should not place the electron beams from the cathodes in exact tangent relation with ux lines, I can produce such exact relation by suitably energizing the ux or convergence control coil 40 by modifying the current input thereto. A determination of an exact tangent relation of the two beams with respect to two flux lines can be made by monitoring the current ouptut of the signal electrode 16 while exposing the photoconductor 18 to light. Maximum signal current will be evidenced when the beams are in tangent relation with lines of magnetic flux. Failure to achieve such tangent relation will cause the beams to cut magnetic flux lines resulting in spinning movement of the beams and consequent failure to land properly on the photoconductor 18. Such failure of the beams to land on the photoconductor results in appreciably reduced signal current.

If the gun assembly 24 should be mounted in the envelope 10 without exact coincidence of its axis 50 with the longitudinal axis of the envelope 10, the beams 104, 106 with zero deflection will not land normal to the photoconductor 18. To correct such failure of coincidence of the gun and envelope axes, the alignment coil 38, placed above the gun assembly 24 as shown in FIG. 1, is suitably energized in a known manner to warp slightly the flux lines in the exit region of the electron beams from the gun to cause the beam trajectories to coincide with liux lines, to prevent spiraling and to cause the beams 104, 106 to land normal on the photoconductor 18. The control produced by the alignment coil 38 affects both beams 104, 106 simultaneously, while the proper convergence of the liux lines 52, 62 for obtaining the simultaneous tangent relation is established by the ilux control coil 40. The iiux control coil 40 only controls the convergence angle of flux lines.

FIG. 4 depicts successive horizontal lines 108, 110 produced when the gun assembly 24 is so oriented that cathode 44 is above cathode 46 and the progression of scanning lines is upwards as viewed in the ligure. During a scanning cycle, line 108 is produced by the reading beam 104 coming from cathode 44. During the succeeding blanking or retrace cycle, a line 110 of much larger thickness is produced by erase beam 106 originating in cathode 46. The thickness of line 108 is about 1 mil, which corresponds to the -diameter of the reading beam 104. The thickness of line 110 is about 30 mils which is the diameter of the erase beam 106. In order to prevent the erasure of information in unscanned areas of the photoconductor 18 immediately above line 108, the beam 110 is spaced below line 108 a distance at least equal to the diameter of beam 106. When the diameter of beam 106 at the photoconductor is about 30 mils I prefer to space the erase line 110 about 50 mils below the reading line 108. This spacing is achieved by spacing the centers of apertures and 102 (FIG. 2) a suitable distance from each other. When the magnification of the electron optics of the tube is one-half, a spacing of 100 mils between the aforementioned aperture centers will result in the preferred spacing of 50 mils between the scanning lines 108, 110 (FIG. 4). In view of the relatively large diameter and current content of the erase beam 106, it effectively erases and completely discharges the portions of the photoconductor 18 scanned by the reading beam 108, at the termination of each scanning line 108. As a consequence, the entire photoconductor 13 is fully discharged at the end of a scanning frame and is free from any residual charges that would adversely affect a succeeding scanning frame.

Conventional scanning and blanking circuits provide successive outputs, which may be connected to a grid of the gun producing the electron beam to be scanned, for successively promoting and blocking electron emission from a cathode of the gun. During a scanning cycle the grid, for example, may be impressed with ground voltage from the scanning circuit to permit electron beam emission. After completion of a scanning cycle, the blanking circuit is caused to impress a blocking voltage on the grid of a value for example of `-30 volts with respect to ground. This voltage blocks emission from the cathode.

I utilize such conventional scanning and blanking circuits in a novel manner. During a scanning cycle, I cause the scanning circuit to impress a voltage directly on the reading beam cathode 44, which may be +20 volts for example. Simultaneously therewith I cause the blanking circuit to impress a positive voltage with respect to ground, such as +60 volts, directly on the erase beam cathode 46. When the grid structure 84 is grounded, the foregoing potentials on the two cathodes will cause the reading beam cathode to emit electrons to form a beam, while emission from the erase gun cathode 46 is suppressed. During an erase cycle, I reverse the foregoing potentials on the two cathodes, 44, 46 by circuit means known in the art so that emission from the reading beam cathode 44 is suppressed while emission from the erase beam cathode is permitted. During both scanning and erase cycles the grid 84 is held at ground potential. Scanning and blanking potentials are fed to the reading beam cathode 44 by means of lead 56, and to the erase beam cathode 46 by means of lead 64. The leads 66, 64 therefore constitute means for impressing the 4desired scanning and blanking voltages on the cathode 44, 46 respectively.

During periods when the voltage on one of the cathodes 44, 46 is negative with respect to the voltage on the other cathode, electron emission from the said one of the cathodes to the said other cathode is prevented by the shield 82 (FIG. 2).

I claim:

1. A device comprising in combination:

(a) an elongated envelope,

(b) two electron guns laterally spaced in said envelope,

(c) a target including a relatively thin photoconductive layer Within said envelope, said layer being characterized by enhanced capacitive lag and reduced lateral conductance for high definition, and (d) a focusing coil external of said envelope and adapted to produce curved magnetic liux lines extending through said envelope in a region adjacent to said electron guns, :l

(e) said electron gus having electron emitting surfaces tilted towards each other so that principal rays of the electron beams therefrom are tangent to said flux lines to prevent spiralling of the principal rays as a result of crossing said ux lines and to insure proper landing of the principal rays on said target.

2. A device according to claim 1 and wherein said electron gus are disposed in one end of said envelope on opposite sides of the longitudinal axis of said envelope and trajectories of electron beams produced by said guns converge and substantially in tangent relation with respect to two of said flux lines.

3. A pickup tube comprising:

(a) an elongated envelope,

(b) a target positioned in one end portion of said envelope,

(c) two electron guns positioned in the other end portion of said envelope for directing electron beams of different diameter to said target,

(d) said two -guns comprising two cathodes, and

(e) a beam defining aperture structure common to both of said cathodes land having two apertures in register with electron emitting surfaces of said cathodes,

(f) means for simultaneously impressing different voltages on said cathodes to cause one of said cathodes to emit electrons through one of said apertures and to suppress electron emission from the other of said cathodes, and

(g) means for generating magnetic ux lines in said envelope for focusing said electron beams,

(h) said guns being positioned so that said electron beams are substantially tangent to said magnetic lines of flux to prevent'spiralling of said electron beams.

4. A pickup device according to claim 3 and wherein a shield is positioned between said emitting surfaces of said cathodes to prevent electron emission therebetween when said cathodes are impressed with said different voltages, said shield being a part of said beam defining aperture structure.

5. A pickup tube comprising:

(a) an elongated envelope,

(b) a target positioned in one end portion of said envelope,

(c) two electron guns positioned in the other end portion of said envelope for directing electron beams of different diameter to said target,

l(d) said two guns comprising two cathodes, and

(e) a beam delining aperture structure common to both of said cathodes and having two apertures in register with electron emitting surfaces of said cathode, said two apertures in said beam delining structure being mutually spaced a distance to cause the scanning lines produced by electron beams passing through said apertures to be spaced a distance at least equal to the diameter of the electron beam of larger diameter, for spacing lines produced by said beams on said target, and

(f) means for simultaneously impressing different voltages on said cathodes to cause one of said cathodes to emit electrons through one of said apertures and to suppress electron emission from the other of said cathodes.

6. A pickup device comprising:

(a) an elongated envelope,

I(b) a target including a relatively thin photoconductive layer within said envelope, said layer being characterized by enhanced capacitive lag and reduced lateral conductance for high definition,

(c) a lirst electron gun in said envelope for producing an electron beam for reading information stored in said photoconductive layer, said electron beam having a relatively small cross-sectional larea for increased resolution,

(d) a second electron gun in said envelope for producing an electron beam of relatively large cross-sectional area for fully erasing information remaining in said photoconductive layer lat the termination of a scanning line, for enhanced operation,

(e) a focusing coil externally of said envelope for focusing said beams, and

(f) means for line sequentially scanning said electron beams,

(g) the period of a scanning line of said second electron gun being appreciably shorter than the period of a scanning line of said iirst electron gun.

7. A pickup device according to claim 6 and wherein said electron guns have common grid and beam limiting aperture plate structures.

8. A pickup device according to claim 6 and wherein said guns are tilted with respect to the longitudinal axis of said envelope for disposing the electron beams thereof substantially in tangent relation with magnetic llux lines produced by said focusing coil.

9. A pickup tube according to claim 6 and wherein said second electron gun includes a grid having an aperture spanned by at least one wire for preserving a desired current density in the electron beam produced by said second electron gun.

10. A pickup device according to claim 6 and wherein the ratio of said relatively large cross-sectional area of the electron beam produced by said electron gun to the period of a scanning line of said beam, is larger than the ratio of the relatively small cross-sectional area of the electron beam produced by said irst electron gun to the period of a scanning line of said last-named electron beam.

11. A pickup device according to claim 6 and wherein said electron guns are positioned on Opposite sides of the longitudinal axis of said envelope.

12. A pickup device comprising:

(a) an elongated envelope,

(b) 1an electron gun assembly in one end portion of said envelope adapted to produce tWo electron beams and having an electron exit end,

(c) a focusing coil adjacent to an external portion of said envelope for focusing said electron beams,

(d) an alignment coil external of said envelope and positioned adjacent to said electron exit end of said gun assembly and spaced from any coextensive relation with respect to said gun assembly, for dis-posing one of said beams in tangent relation with respect to a flux line produced by said focusing coil, and

(e) a convergence control coil adjacent to another external portion of said envelope and coextensive with a portion of said electron gun assembly for adjusting the convergence of flux lines produced by said focusing coil to simultaneously cause both of said beams to be tangent to flux lines produced by said focusing coil to prevent said beams from spiralling.

References Cited UNITED STATES PATENTS 2,967,969 l/1961 Stocker 315-13 X 3,164,743 1/1965 Koda et al. 3,278,780 10/1966 Carnahan et al 315-12 X RICHARD A. FARLEY, Primary Examiner.

JEFFREY P. MORRIS, Assistant Examiner.

U.S. Cl. X.R. 315-13, 31 

